I put on over 38kg during my pregnancy... Some of it can probably be attributed to the fact that I was unhealthily underweight before pg, plus I quit smoking in the month after my BFP. Also the only way for me to curb M/S was to eat... constantly...
So I would advise eating HEALTHILY and to NOT overeat (more than what you really need) or be too greedy ( like I was, LOL!)
Gentle excercise is a great way to keep the extra kg's off too. I noticed my rate of weight gain really slowed when I started doing 2 Pregnastics classes a week.

Anyway, back on the topic of losing weight after pg. Someone mentioned above there is a supplement you can get from the chemist. Well it is called Optifast... it used to only be available on script, but that has changed.
I have been using Optifast instead of one of my meals each day (lunch or dinner) for almost 3 weeks now, and I have already lost around 3 kgs, YAY!!